Reaction injection molded (RIM) urethane, a thermosetting plastic, has been widely used in recent years as a material of construction for exterior automotive parts, such as bumpers and other automotive body and trim parts. RIM urethane is a relatively inexpensive, easy to mold, and temperature resistant material with good flexibility and impact strength, properties which are desirable for exterior automotive applications. However, RIM urethane, because of its high aromatic isocyanate content, is very light sensitive and rapidly degrades and yellows when exposed to U.V. light. Furthermore, the degradation by-products tend to migrate up into the topcoat finish, marring its aesthetic and protective properties. Yellowing, chalking, fading, cracking, and delamination of topcoats are common effects of RIM degradation.
Protective primers have been developed in the past for application beneath automotive topcoats, in order to inhibit U.V. light penetration into the underlying RIM substrate and prevent RIM degradation and by-product migration. Current RIM primers in use today are typically formulated as sprayable, high solids, solvent borne compositions based on polyester/melamine resin binders, carbon black pigments, U.V. absorbers, and catalysts, together with other conventional additives, such as mar/slip additives, flow control agents, antioxidants, etc.
Yet, the primers today must be coated over the RIM urethane at high dry film thicknesses, typically ranging from 20 to 25 microns (0.8 to 1.0 mils), to provide adequate UV protection. High film builds are, however, undesirable from the standpoint that more primer material must be applied, which, in turn, increases material costs as well as volatile emissions. In the past, it has not been possible to improve U.V. light protection at lower film builds by increasing the levels of U.V. light-blocking carbon black pigments in the primers. Prior attempts to increase carbon black loadings to above current levels (i.e., pigment to binder ratio of 0.04) have generally been unsuccessful. The resultant primers have proven unworkable, since the increased amount of carbon black tends to increase the viscosity to levels where the primer cannot be sprayed through standard spray equipment.
Current primers also require a separate cure cycle before topcoat application, typically performed in a separate bake for 20 to 30 minutes at 250.degree. F., in order to achieve good initial appearance and adhesion. Yet, in its uncured state, the primer coat generally lacks the ability to holdout the topcoat if applied direct thereover. The topcoat would, in turn, tend to migrate and sag down into primer, marring topcoat appearance and physical properties. This prevents the use of a so-called "wet-on-wet" coating technique, wherein one coating is coated directly over another uncured coating followed by curing the multi-layered coating together in one step without the need for a separate bake in between.
More recently, wet-on-wet coating techniques have been practiced with the automotive topcoats, wherein a first topcoat layer, such as a color basecoat, is initially applied over a cured primer layer, then overcoated wet-on-wet with a second topcoat, such as a clear overcoat, and finally cured simultaneously in a single topcoat bake. The primer is not able to participate in the wet-on-wet coating process and must be separately baked prior to topcoat application. Separate bakes, however, are undesirable, since they result in higher production times and energy costs.
U.V. protection at low film builds and wet-on-wet coating with topcoats have not heretofore been possible with current primers, which contributes to the cost of painting RIM urethane parts. This has led the automotive industry to use replacement substrates, such as thermoplastic olefins, having both lower processing and material costs. It would be desirable to provide a primer composition which can overcome the foregoing drawbacks, so as to make the use of RIM urethane substrates for automotive applications once again attractive to the industry.
What is needed is a primer composition that is suitable for coating automotive parts, such as RIM urethane substrates, that can form a U.V. barrier coat which inhibits U.V. light penetration at significantly lower film builds to insure long term adhesion, and which prevents underlying substrate degradation and the resultant by-products from migrating up into the topcoat, and that also can be applied wet-on-wet with a topcoat and cured simultaneously in the topcoat bake without detracting from the topcoat appearance, adhesion, and U.V. protection.